This invention relates to an integrated circuit having a PNP bipolar electronic device.
In particular, the present invention relates to a lateral PNP bipolar electronic device which is integrated monolithically on a semiconductor substrate together with other bipolar devices of the NPN type.
As is well known, preference is given to NPN transistors for implementing integrated circuitry of the bipolar type on semiconductor substrates, on account of their direct and alternating current amplifications being definitely better than those of PNP transistors.
Another disadvantage of PNP bipolar transistors is their close limitations in high frequency applications.
Despite all this, the inclusion of both NPN and PNP transistors on a common semiconductor substrate is still a necessity where suitable bias circuits, current mirror circuits and/or load devices for gain stages are to be provided.
The formation of PNP transistors on a semiconductor substrate is effected concurrently with that of NPN transistors, without any additional implanting or masking steps.
It is for this reason that it has become common practice to form PNP bipolar transistors of the so-called lateral type such that they can be compatible with the process flows adopted to fabricate NPN bipolar devices.
In this respect, FIG. 1 shows an enlarged cross-sectional view, taken on a vertical plane, of a lateral PNP device 6 formed on a P-type semiconductor substrate 1.
The following are successively deposited onto this substrate 1: a first buried layer 2 doped N+ to form the base region of the transistor, and a second layer 3, doped N, which constitutes the active area of the PNP device to be.
Thereafter, a selective diffusion of P-type dopants is effected in the active area 3 to define a central emitter region 4 surrounded by collector 5.
This solution has been widely used heretofore, to the point that a text, xe2x80x9cDesign and Realization of Bipolar Transistorsxe2x80x9d, Peter Ashburn, page 157, gives it as the principal configuration for a lateral PNP device that can ensure of a good current gain.
It should be noted that such lateral PNP transistors have an intrinsic current loss of about 3% compared to their respective collector currents; these currents can also be explained theoretically by having reference to the diagram in FIG. 2 of an equivalent electric circuit of a lateral PNP transistor.
It can be seen in FIG. 2 that the lateral PNP device, designated Q1 in the equivalent circuit, is connected to a pair of parasitic PNP transistors Q2 and Q3 having their emitter regions connected to the emitter and collector regions, respectively, of the transistor Q1.
In addition, these parasitic devices Q2 and Q3 have their collector regions connected to the semiconductor substrate, and their base regions in common with the base region of Q1.
This equivalent electric circuit diagram shows that the parasitic currents are mainly attributable to the second parasitic device Q2 draining toward the substrate some of the current being injected by the emitter of Q1, thereby lowering the efficiency of lateral emission.
Similar considerations apply to the third parasitic device Q3, which contributes instead to lowering the collection efficiency of the collector of Q1 when the latter is biased into its saturation range.
To overcome drawbacks of this kind, tied to the parasitic currents, a conventional solution has been that of optimizing the efficiency of collection of the carriers in the collector region, so as to maximize the gain of lateral transistors.
For this reason, lateral PNP transistors have been implemented conventionally with their emitter region occupying, in the active area, a central position surrounded by two collector regions.
While being in many ways advantageous, this prior solution has a serious drawback in that it leads to increased values of certain characteristic parameters of the lateral PNP transistor, such as the base-collector and emitter-base capacitances, Cbc and Ceb, which restrict performance at high frequencies.
This drawback is also connected with the considerable spread of the base region in the buried layer brought about by the need to control the lateral PNP device performance in the emitter and collector regions.
Particularly, a width Wb of this base region adversely affects the carrying parameter B*, which is tied to the Wb by the following formula:                               B          *                ≈                  1          -                                                    xe2x80x83                            ⁢                              Wb                2                                                    2              ⁢              D              ⁢                              xe2x80x83                            ⁢              τ                                                          (        1.1        )            
where D is the diffusion value, and t is the re-combination time of the carriers.
It should be noted that the B* factor is inversely proportional to a passage time through the base region, so that as it increases, the frequency performance of the lateral PNP device deteriorates.
Unfortunately, this type of lateral PNP device reveals serious limitations when attempts are made to improve its high frequency performance.
These limitations come from the practical impossibility of bringing the collector regions closer to the central emitter region.
This is both attributable to the photolithographic masks used for transferring the patterns of the active region, and to side diffusion effects during the formation of the emitter and collector regions, as well as to breakdown effects which may occur in a region between the base and the collector.
Consequently, the width Wb of the base region of the lateral PNP transistors always exhibits values between 2 xcexcm and 4 xcexcm.
FIG. 3 is a vertical cross-section view, to an enlarged scale, of a conventional lateral PNP transistor, highlighting polysilicon contacts 10 above the emitter 4 and collector 5 regions.
These contacts 10 are characterized by the presence of so-called xe2x80x9cbird""s beaksxe2x80x9d which jut out sideways from the emitter and collector regions. The possibility of altering the width Wb of the base channel is usually hindered by the bird""s beak protrusions.
In fact, calling WPPL the distance between the emitter and the collector, LBB the reach of the bird""s beak, and D the misalignment between the polysilicon contact layer 10 and the active area, then:
WB greater than WPPL+2*LBB+2*D
And substituting for some real sample values of such distances, we get:
WBxe2x89xa61.0+2*0.5+2*0.4=2.8 xcexcm
WBxe2x89xa61.0+2*0.5+2*0.15=2.3 xcexcm.
Thus, the heaviest restriction on the reduction of the base channel width appears to be due to the process implementation, rather than to its topography.
From FIG. 4, showing a schematic detail view of FIG. 3, it is evinced that the bird""s beak partly overlaps the active area and that a window through which the implantation of BF2 is effected is reduced accordingly.
This results in the creation of a P+-N junction within the polysilicon layer deposited, which keeps the base and collector currents far from the ideal.
In light of the foregoing considerations, an object of this invention is to provide a new topography conferring, on a lateral PNP device, such structural and functional features as to make it suitable for high frequency applications as well, thereby overcoming the aforesaid limitations and drawbacks.
Another object of this invention is to define the width of the base channel by way of the emitter structure. Particularly, an object of this invention is to reduce substantially the width of the base channel. In fact, a reduction in the base channel width would ensure an improved value of the carrying factor B*.
These and other objects of the present invention can be accomplished by providing a lateral PNP bipolar transistor having a substrate doped with impurities of the P type, a first buried layer doped with impurities of the N type to form a base region, and a second layer, overlying the first layer and having a conductivity of the N type, to form an active area with collector and emitter regions being formed in said active area and separated by a base channel region wherein a width of the base channel region is defined essentially by a contact opening formed above an oxide layer deposited over the base channel region.